The present invention relates to a display apparatus provided with a plurality of light emitting devices such as light emitting diodes arrayed in a matrix display panel, and to its method of operation.
Currently, bright red, green, and blue (RGB) light emitting diodes (LEDs) of 1000 mcd or more have been developed, and fabrication of large-scale LED displays has become possible. These LED displays have features such as low power consumption, lightness in weight, and the possibility for thin panel display. Further the demand for large-scale displays, which can be used outdoors, has increased dramatically.
Practical large-scale LED displays are configured to fit the installation space by assembling a plurality of LED units. An LED unit is formed from a dot matrix array of RGB LEDs arranged on a substrate board.
Further, an LED display is provided with a driver circuit capable of driving each individual light emitting diode. Specifically, each LED control device, which transmits display data to each LED unit, is connected to the LED display, and a plurality of LED units are connected to form one large-scale LED display. The number of LED units used increases as the LED display becomes larger in scale. For example, a large-scale display can use 300 vertical×400 horizontal, or 120,000 LED units.
The LED display uses a dynamic driver system as its driver method, and specifically, the display is connected and driven as described below.
For example, in an m×n dot matrix LED unit, each LED anode in each line is connected to a common source line, and each LED cathode in each column is connected to a common current line. The m-line common source lines are sequentially turned on for display with a prescribed period. For example, m-line common source line switching is performed via decoder circuitry based on the address signal.
However, when LEDs connected to a selected common source line were activated in related art apparatus, charge accumulated in non-activated LEDs connected to unselected common source lines. When these common source lines were then selected, excess current developed as a result of charge built-up during their inactive period. As a result of this problem, LEDs controlled to be off emitted low levels of light and sufficient image contrast could not be obtained. These types of effects caused display quality degradation.
Thus, the first object of the present invention is to reduce the effects of accumulated charge and provide a high quality image display apparatus and its method of operation.
Further, in an LED display, corrected image data are typically used for each LED device to display a high quality image. This is because device-to-device LED variation in brightness, for example, is relatively large.
More specifically, the control circuit has a read-only-memory (ROM) correction data memory section to store correction data corresponding to each LED device. Corrected image data based on the correction data stored in ROM has been used for display.
However, since correction data were stored in ROM in related art apparatus, correction data could not be re-written. Consequently, related art apparatus had problem in that it was necessary to provide a re-writable memory device separate from the ROM when different correction data were required.
Thus, the second object of the present invention is to provide an image display apparatus which can store a plurality of correction data in one correction data memory section.
Further, to accurately represent image data on an LED display, the light emission characteristics (driving current vs. brightness characteristics) of each LED device in the image display apparatus must be uniform. However, since LEDs are fabricated on wafers by semiconductor technology, light emission characteristic variation results from fabrication lot-to-lot, wafer-to-wafer, and chip-to-chip. Therefore, it is necessary to correct image data amplitude to compensate for light emission characteristic differences of the LED for each pixel.
An example of related art image data correction is described as follows.
Turning to FIG. 12, a block diagram of an embodiment of a related art LED display is shown. In FIG. 12, 101 is an m-line n-column LED matrix, 107 is a control circuit, 105 is a microprocessor unit (MPU), 106 is a ROM to store correction data, 102 is a common driver circuit, 103 are horizontal driver circuits, 109 are correction circuits to correct image data, and 110 are random access memory (RAM) to temporarily store correction data. The horizontal driver circuits 103, correction circuits 109, and RAM 110 are integrated in LED driver integrated circuits (IC's) 104 provided for each column of the LED matrix (k=1 to n).
First, prior to display illumination, correction data for the m×n pixels stored in ROM are transferred to a high speed buffers. RAM 110 are used as the high speed buffers. Correction data transfer is accomplished as follows. First, correction data held in ROM 106 are read out by the MPU 105. The MPU 105 sequentially selects LED driver IC's 104 (k) via the address bus 111 and sequentially outputs one columns-worth, or m-pixels, of correction data corresponding to each selected column. The correction data output is input to each LED driver IC 104 (k) via the correction data bus 112 and stored in RAM 110 internal to the LED driver IC 104 (k).
When LEDs are illuminated, correction data stored in RAM 110 are sequentially read out by correction circuits 109. The value of input image data (IMDATA) is increased or decreased for each pixel based on the correction data to achieve image data correction. Corrected image data are output to the driver circuits 103, and the driver circuits 103 produce driving current for each LED based on the corrected image data.
However, in the related art LED display described above, a total of m×n pixels-worth of correction data must be stored in the buffers, or RAM 110, and as display pixel count increases, very large RAM capacity becomes necessary. Further, the operation of correction data read-out from RAM 110 to the correction circuits 109 becomes complicated as the amount of RAM increases. In addition to these problems, both the address bus 111 and the data bus 112 must branch to, and connect with, each of then driver IC's 104(1 to n), thereby making wiring complex and peripheral circuitry large in area.
Thus, the third object of the present invention reflects consideration of these problems, and is to provide an image display apparatus which can reduce the amount of data stored in the buffers, and can accomplish image data correction with a simple circuit structure.
The above and further objects and features of the invention will be more fully apparent from the following detailed description with the accompanying drawings.